By Authority Of
THE UNITED STATES OF AMERICA
Legally Binding Document
By the Authority Vested By Part 5 of the United States Code § 552(a) and
Part 1 of the Code of Regulations § 51 the attached document has been duly
INCORPORATED BY REFERENCE and shall be considered legally
binding upon all citizens and residents of the United States of America.
HEED THIS NOTICE '. Criminal penalties may apply for noncompliance.
Document Name: ICS: Clean Seas Guide for Oil Tankers
CFR Section(s): 33 CFR 157 23(b)
Standards Body: international Chamber of Shippin
International Chamber oi Shipping
Oil Companies International Marine Forum
Clean Seas Guide For Oil Tankers
The Operation of Load on Top
S/S CHEVRON CALIFORNIA
International Chamber of Shipping
Oil Companies International Marine Foram
S/S CHEVRON CALIFORNIA
Clean Seas Guide For Oil Tankers
The Operation of Load on Top
Published Jointly by
International Chamber of Shipping
Oil Companies International Marine Forum
Printed in England
Witherby & Co. Ltd., London
FOREWORD -- ■■ 1
I PROCEDURES— THE BASIC METHOD . . 5
(A) Basic Method Assumptions . . . . . . . . . . . . 5
(B) Sequence of Procedures .. ., .. .. .. .. 5
(C) Line Draining and Taking On Dirty Ballast 5
(D) Tank Washing 6
(E) Loading Clean Ballast 6
(F> Settling of Dirty Ballast 6
(G) Disposal of Dirty Ballast 6
(H) Slop Tank Discharge . . . . . . . . . . . - • • 7
(I) Final Line and Pump Flush . . . . . . . . . . . • 8
(J) Discharging Clean Ballast . . . . . . . . . . . . 8
(K) Disposal of Slop Tank Residues . . . . . . . . . . 9
(L) Handling Sludge 9
II OPERATIONAL FACTORS INFLUENCING BASIC METHOD 10
(A) Short Haul Voyages 10
(B) Coastal Voyages 10
(C) OBO and Ore/Oil Carriers 10
(D) Preparation for Repair Ports . . . . . . . . - - 11
III OTHER FACTORS INFLUENCING BASIC METHOD .. 12
(A) Insufficient Slop Tank Capacity .. .. .. .. .. 52
(B) Multiple Slop Tanks 12
(C) Continuous Flow Decanting . . . . . . . . . . 13
(D) Eductors — Open Cycle
Full Recirculation . . . . . . . . . . 13
(E) Slop Tank Heating Coils . . . . . . . . . . . . 13
(F) Oii/Water Separators 13
(G) Oil Content Meters 14
(H) Oil/Water Interface Detectors 14
(I) Tank Cleaning Chemicals .. .. .. .. .. .. 15
(J) Demulsifiers . . . . . . . . . . . . . . 15
IV RESPONSIBILITIES 15
The Clean Seas programme, introduced by the oil and shipping industries
in 1964 to restrict the operational discharge of oil into the sea, incorporated
the now well-known concept of Load on Top.
This Guide, which describes the Load on Top procedures, is designed to
help shipboard personnel to comply with the oil discharge limits contained in
the 1969 amendments to the 1954 International Convention for the Prevention
of Pollution of the Sea by Oil.
Effective Load on Top operation calls for complete understanding of the
problem, strict adherence to the procedures, and vigilant supervision. Success
depends upon full co-operation, from all to whom this Guide is addressed:
the shipowner, who must ensure that all his ships are operated in
accordance with the recommended procedures;
ships' personnel — both officers and crews— who must ensure that in-
structions are strictly complied with;
oil companies and others upon whom rests the ultimate responsibility for
avoiding sea pollution; and
cargo shippers and receivers, who are also involved in these procedures.
The oil and shipping industries, through the Oil Companies International
Marine Forum and the International Chamber of Shipping, strongly commend
this Guide to all concerned.
Not all oil pollution is caused by tankers. However, the lruge volume of
crude oil transported by sea has created a major problem in disposing of dirty
ballast and tank washings without harming the marine environment or damaging
It is this problem, associated with all persistent oils, with which this
Guide is concerned.
Most crude oils contain wax and other materials in solution, together
with sediments, which may settle out during the voyage and form a residue with
any cargo remaining after discharge (of the order of 0.2 to 0.5 per cent of the
cargo carried). If discharged into the sea in heavy concentrations in the course
of tank washing the residue will stay on the surface for a long time and cause
Hence it must be kept on board and may be discharged at loading terminals
where the necessary facilities are available. If not, the residue must be retained
and the new cargo loaded on top. In no circumstances may the residue be dis-
charged into the sea unless the safety of the ship or its personnel is at hazard.
Objects of Load on Top
The essential purpose of the Load on Top system is the collection and
settling on board of the water and oil mixtures resulting from ballasting and
tank operations — usually in a special slop tank or tanks — and their subsequent
disposal ashore at the discharge port.
When oil and water are agitated together droplets of oil can enter the
water and water can enter the oil. When oil droplets enter water they are generally
well dispersed and will settle out, the rate depending upon the specific gravity of
the oil and the size of the oil droplets.
Tank washing and pumping of wash water and oil residues produce
water droplets which will enter the oil. Most crude oils contain emulsifying
elements which hinder the separation of water dispersed in the oil. The oil
residues in the slop tank generally contain this type of water-in-oil emulsion
which is stable and long-lasting.
The 1954 Convention as Amended
All tankers can perform Load on Top operations as described in this
Guide, which will enable them to meet the requirements of the 1954 Convention
as amended in 1969.
The 1969 amendments impose a total prohibition on the discharge of
any oil or oily mixture from a tanker within 50 miles of any coast, and limit the
flow, concentration and quantity discharged anywhere else at sea.
Limited discharges may be made when the following conditions are
(i) "The tanker is proceeding en route".
(This eliminates the possible concentration of the permitted dis-
charge from a stationary vessel or from a vessel steaming in tight
circles, and ensures that the discharge is well-distributed.)
(ii) "The instantaneous rate of discharge of oil content of any effluent
shall not exceed 60 litres per mile".
(The "instantaneous rate" means the rate of discharge of oil content
in litres per hour at any instant divided by the speed of the tanker
in knots at the same instant. Experiment has shown that 60 litres per
mile discharged in this way disperses rapidly and leaves no trace of
oil on the surface of the water within 2 or 3 hours of discharge.)
(iii) "The total quantity of oil discharged on a ballast voyage does not
exceed 1/15,000 of the total cargo-carrying capacity".
(This puts a limit on the total amount of oil that can be discharged
even at the reduced rate set out in (ii).)
(iv) "The tanker is more than 50 miles from the nearest land".
(This is to make quite certain that the very small amounts of oil
permitted to be discharged in compliance with requirements (ii) and
(iii) will not reach the shore.)
Water effluent cleanliness is fiuther discussed in the Annex.
These restrictions, all of which must be complied with, apply to crude oil,
fuel oil, heavy diesel oil and lubricating oil. Discharges from pump-room bilges
should be treated in a similar manner.
When a tanker is within 50 miles of land the only permitted discharge is
clean ballast from a cargo tank which has been so cleaned that the effluent
therefrom would produce no visible traces of oil on the surface of the water.
In the case of oil or oily mixtures from machinery space bilges, the re-
quirements in (i) and (ii) above must be satisfied and additionally the oil content
of the discharge must be less than 100 parts per 1 million parts of mixture, and
made as far as possible from land. While there is no specified limit to the amount
or the distance from land of the oil discharged from machinery space bilges,
there is an absolute ban on the discharge of any type of oil in some territorial
I PROCEDURES— THE BASIC METHOD
(A) Basic Method Assumptions
The procedures described in this Section assume that the tanker is using
stripping pumps and has a single slop tank. This is the basic Load on Top
method. Variations on this method entailing the use of eductors or more com-
plex slop tank arrangements are described in Section III.
(B) Sequence of Procedures
(a) Thoroughly drain cargo tanks and lines on completion of discharge.
(b) Take on dirty ballast, thoroughly flushing cargo lines and pumps initially.
(c) Wash cargo tanks and collect oily wash water in the slop tank.
(d) Flush cargo lines where necessary.
(e) Take on clean ballast.
(f) Allow dirty ballast to settle.
(g) Discharge "clean" part of dirty ballast to sea beyond 50 miles from nearest
(h) Strip "dirty" part of dirty ballast to slop tank.
(i) Flush the stripping system.
(j) Settle and decant water from slop tank.
(k) Discharge clean ballast
(1) Dispose of slop tank residues.
(C) Line Draining and Taking On Dirty Ballast
1. All lines containing cargo should be pumped ashore on completion of
discharge. This involves opening cross connections between cargo and
2. Any remaining cargo in the lines which cannot be pumped ashore should
be drained into a separate tank or into the tank chosen as the slop tank
before ballasting begins. Where the ship has no special slop tank the
aftermost centre cargo tank is frequently used for this purpose.
3. Dirty ballast should then be taken on, making sure that a pump is started
before opening the main sea valve. In the process all cargo lines should
(D) Tank Washing
4. Cargo tanks should next be washed and contents stripped to the slop
tank. Care should be taken to ensure that wash water does not build up
on the tank bottom.
5. It should be understood that the quantity of oil discharged to the sea
can be minimised by eliminating unnecessary washing, thus minimising
the quantity of water brought into contact with oil within the tanker.
6. As the clean ballast is to be discharged directly to the harbour it is im-
perative that the tanks in which it is carried are clean.
(E) Loading Clean Ballast
7. Before starting to load clean ballast make sure that the main cargo pumps
and lines are clean by further flushing into a suitable dirty ballast tank
or slop tank.
(F) Settling of Dirty Ballast
8. A good oil/water separation of the dirty ballast takes time, depending
upon the movement of the ship as well as on the previous type of oil
cargo carried. Under favourable conditions it may take no more than 12
hours. But in most circumstances 24 hours or more are needed to achieve
(G) Disposal of Dirty Ballast
9. When settling is complete the ballast tanks will contain water with an oil
layer on top. The bulk of this water may now be discharged to the sea,
preferably when the vessel is steady, applying the following procedures:
(a) If necessary, flush main cargo lines and pumps into a dirty ballast
tank or slop tank.
(b) Start to discharge ballast water.
(c) Reduce discharge rates from individual tanks on approaching a
water depth of about 15 per cent of the tank depth.
(d) Slow down pumping rates to avoid drawing surface oil into the
suction by vortex or weir effects.
(e) Stop discharge of individual tanks when a level has been reached
which, for the particular ship, is known not to give rise to any en-
trainment of oil. When all tanks have been discharged to this level
all discharge overboard must cease.
(f) At this stage the officer in charge should verify that the slop tank can
take the volume of dirty ballast remaining. If ullage is insufficient,
then the slop tank may be partially discharged to provide the neces-
sary capacity, taking care to ensure that an adequate depth of water
remains beneath the oil residue layer.
(g) Start transferring the remaining dirty ballast into the slop tank,
using the stripping system.
(h) Before final clean-up, transfer to the slop tank the contents of the
pump room bilges and any other bilges connected to the cargo
(i) The stripping system will then be dirty and should be flushed into
the slop tank.
(H) Slop Tank Discharge
10. Decanting of the slop tank contents is a critical step in the Load on Top
1 1 . Hence the timing of the various steps in the operation is important. Even
a short delay in stopping a pump or closing a valve can allow a significant
quantity of oil to escape into the sea.
12. As with dirty ballast, the time required for oil and water to separate in
the slop tank depends upon the motion of the ship as well as on the type
of previous cargo.
13. Under favourable conditions a few hours may be enough, but longer
periods are desirable. In most circumstances, 36 hours or more should be
14. Before starting discharge an accurate interface and ullage reading must
be taken to determine the depth of the oil layer.
15. The interface profile may vary over several inches in depth. Hence, dis-
charge from the tank must cease well before the measured interface is
reached to avoid discharge of any oil-in-water emulsion overboard.
16. Although every effort should be made to remove as much water as
possible from the slop tank, the prime objective is to prevent oily water
reaching the sea.
17. Extreme care is therefore necessary, and the overboard discharge must
be closely checked.
18. Agitation of the slop tank contents must be kept to a minimum to avoid
drawing surface oil into the suction by vortex or weir effects, particularly
as the oil/water interface approaches the top of the structural members
in the tank bottom. Pumping rates must be strictly controlled.
The following detailed procedures should be followed:
(a) Pump down the slop tank using one main cargo pump at slow speed
until a water depth of about 1 5 per cent of the tank depth is reached.
(b) Stop the cargo pump, then take an oil/water interface and ullage
reading and re-calculate the remaining water depth.
(c) Resume pumping of the slop tank, this time using a stripping pump,
until a predetermined water depth is reached which, for the par-
ticular size and construction of the slop tank, is known not to give
rise to discharge of oil. Pumping, which may initially be at a moderate
rate, should be slowed as this predetermined water depth is ap-
proached. Check visually (or by instruments) for any signs of oil in
the water issuing from the stripping pump drain cock.
(d) If oil should appear before the predetermined water depth is reached,
(e) Should this occur, further settling of the slop tank contents should
be allowed for as long as possible before repeating steps (c) and (d).
(f) No decanting must take place beyond the predetermined water depth.
(I) Final Line and Pump Flush
19. As an added precaution, after these operations have been completed and
while the ship is still more than 50 miles from any coast, the lines and
pumps which will be used to discharge clean ballast must be thoroughly
flushed to sea.
(J) Discharging Clean Ballast
20. If the tank washing has been carried out as described, there should be no
risk of any oil being discharged overboard in the loading port. Neverthe-
less, the ballast outlet should be watched, particularly when draining a
tank bottom, as this is the most likely time for oil to be picked up. If this
happens discharge must be stopped immediately and the remainder in
the tanks stripped to the slop tank.
(K) Disposal of Slop Tank Residues
21. Before reaching the loading port the master should advise his owners or
charterers of the amount of retained residues on board. These may be
disposed of in several ways :
(a) Pump the residues ashore at the loading terminal, subject to reception
facilities being available.
(b) Retain the residues on board and load the new cargo on top of them.
(c) Retain the residues on board, but segregated from the new cargo.
If this is done, it may be possible to pump them ashore at the dis-
charge terminal, if reception facilities are available. It may, however,
be necessary to carry them forward for more than one voyage.
(L) Handling Sludge
22. Only small quantities of sludge should remain in the cargo tanks after
23. Most of the sludge that accumulates on tank bottoms and internal
structures can be stripped to the slop tank and discharged later, together
with the next cargo or to shore reception facilities.
24. Should areas of heavy sludge build-up be found after initial tank washing,
they are better dealt with by specially locating tank washing machines
and re-washing with very hot water than by immediately resorting to
25. In this manner much of the sand, scale, sediment and oil can be transferred
to the slop tank, thereby minimising removal by hand.
26. The small amount of sludge that is found after efficient cleaning is com-
posed principally of scale, sand, water, wax and sediments of various
27. When sludge containing oil or oil residue is recovered by hand, the
material should be retained on board in suitable containers and disposed
28. Scale or sediment that clearly has no oil content may be dumped directly
overboard. If the presence of oil in the sludge is in doubt, however, it
must be kept on board.
II OPERATIONAL FACTORS, INFLUENCING BASIC METHOD
(A) Short Haul Voyages
1 . The Load on Top operation takes time. The fact that tankers on short
haul voyages have only .limited time is no excuse for curtailing or by-
passing the procedures if this results in oil being discharged into the sea.
Several possibilities are available:
(a) Eliminating tank washing and discharging dirty ballast at the loading
(b) Washing ballast tanks at the discharge terminal and sailing with
(c) Segregating ballast in some cargo tanks as a temporary trading
(d) Re-routeing the ship or slowing down to allow enough time for
proper Load on Top procedures.
(B) Coastal Voyages
2. If a tanker's passage between her discharge port and the next loading port
is wholly within 50 miles of the coast, no discharge associated with a
Load on Top operation is permitted.
3. All dirty ballast and cargo tank washings must be discharged at the
loading port if reception facilities are available.
4. Otherwise, tanks must be cleaned alongside the discharge terminal
(provided it has reception facilities); some cargo tanks temporarily
assigned to segregated ballast; or the ship diverted to a port with the
necessary shore reception facilities.
5. If a Load on Top operation must be carried out, then the ship must go
beyond the 50-mile limit to decant the water from the ballast and slop
(C) OBO and Ore/Oil Carriers
6. For the purpose of this Guide, OBO and Ore/Oil carriers when engaged
in the carriage of oil can be considered as tankers, and therefore in the
main the basic Load on Top method applies to them.
7. However, these vessels are generally equipped with two inter-connected
slop tanks. Variations on the basic method using more than one slop
tank are described in paragraph (B), page 12.
8. OBO and Ore/Oil carriers transferring from oil to dry bulk trades are
obliged to clean the entire cargo system. The large quantities of oil
residues resulting have subsequently to be disposed of ashore.
9. Various means for the disposal of these residues are open to the owner
(a) All residues are retained on board, and discharged ashore at the
(b) If reception facilities are not available at the loading port then a
deviation can be made to a port where these facilities exist.
(c) If (a) and (b) are not possible, all residues are retained on board,
thereby incurring deadweight on the loaded passage.
(d) The vessel is cleaned before departure from the discharge port and
all washings are discharged ashore.
1.0. Operators of OBO and Oil/Ore carriers are particularly reminded of
safety regulations in some dry bulk cargo loading and discharging ports
relating to vessels having oil residues on board.
(D) Preparation for Repair Ports
11. A more intensive tank cleaning programme carried out in advance of
the repair period will reduce the amount of oily residue and sludge which
has to be removed from cargo tanks on the voyage to the repair port.
12. Some crudes help to dissolve sludge, and it is useful where practicable to
carry a cargo of such a crude on the last voyage before the ship goes in
13. Several options are open to owners with ships cleaning for repair, de-
pending upon the length of the voyage and the ship's route from her
(a) Cleaning ballast tanks at discharge terminal and pumping the tank
washings to shore reception facilities.
(b) Performing a basic Load on Top operation and retaining the residues
for discharge to shore reception facilities at the repair port or inter-
(c) Using a tank cleaning berth either at the repair port or intermediate
port for all or part of the cleaning operation.
14. Owners should ensure that the port has shore reception facilities with
enough capacity to take the slops collected on board, together with the
washing water necessary to clean the slop tank. Before arrival the Master
should indicate how much dirty ballast water (if any) the ship has on
board, the grade of the last cargo, and the amount of oil residues and
sludge for disposal.
15. Where repair port reception facilities are inadequate or non-existent,
permission may be given to inert the slop tank during the repair period.
On putting to sea the retained slops will be processed by the basic method.
Ill OTHER FACTORS INFLUENCING BASIC METHOD
The basic method may have to be modified because of shortcomings in
existing equipment, or because the ship has additional equipment not hitherto
discussed. The variations in procedure are discussed below:
(A) Insufficient Slop Tank Capacity
In this eventuality tank washing will have to be interrupted to allow time
for the proper settling and decanting of part of the slop water content
(see paragraph (H), page 7).
(B) Multiple Slop Tanks
Two interconnected slop tanks will provide additional stages in the gravity
separation process. The primary tank will be allowed to fill with tank
washings and residues until a level is reached at which the liquid will
start to run into the secondary tank.
The first stage of separation will then have taken place, with oil floating
on the surface of the primary tank, and either clean water or water with
a low oil content flowing into the secondary tank.
The secondary tank will be allowed to fill to about 80 per cent of capacity.
An oil/water interface should then be determined and the water content
run into the sea. Some tankers may have a third slop tank, enabling a
further stage of gravity separation to be used. Provision of multiple slop
tanks may enable continuous flow decanting to take place.
(C) Continuous Flow Decanting
By this method the slop tank continuously discharges by gravity while
simultaneously receiving cargo tank shippings. Since oil/water separation
takes time and is affected by tank and piping configuration, this method
is not recommended unless the system has been verified as producing
an acceptable discharge.
The use of eductors on open cycle is not recommended. Drive water is
taken from the sea, and the large quantities which have to be processed
may exceed the holding capacity of the slop tank.
The more water used, the greater the total oil outflow, even in low
The eductor drive water is drawn from the bottom of the slop tank while
water direct from the sea is used for cargo tank washing. Hence the total
quantity of water will be less than for open, cycle and may eliminate the
problem of insufficient slop tank capacity.
AH the water required for cargo tank washing and eductor drive is drawn
from the bottom of the slop tank. Hence the total amount of water used
is limited to that pre-loaded into the slop tank. If the tank atmosphere is
uncontrolled during the washing process then internationally accepted
safety recommendations apply.
(E) Slop Tank Heating Coils
Heating coils may be installed suitably located to heat and separate water
in the emulsion layer.
(F) Oil/Water Separators
Separators may be fitted to process dirty ballast water and settled water
from slop tanks. They should be capable of high throughputs.
Most separators are of the gravity type, relying upon the difference in
density between oil and water for separation. They are capable of re-
ducing the oil content of water to be discharged overboard, but the
standard of separation achieved depends to a large extent upon the
pumping rate into the separator. Separators are designed to separate oil
droplets from water; they will not separate oil and water from stable
water-in-oil emulsions. Because the specific gravity of these emulsions
approaches that of sea water, there is a risk that the emulsion could be
forced over into the clean water outlet.
(G) Oil Content Meters
Fitting an oil content meter (or trace detector) in an overboard discharge
line permits continuous monitoring of the quality of the water being
discharged. It may be equipped with visual or audible warning signals to
assist the operator.
(H) Oil/Water Interface Detectors
Oil/water interfaces are usually located by the use of a modified ullage
tape which works on the principle that salt water conducts electricity.
Current is produced electrolytically by the difference in electric potential
between a zinc insert in the tape weight and the steel structure of the tank.
No batteries are required. If no deflection of the meter needle occurs
even when the zinc tip is known to be well down into the water layer, this
may be due to poor contacts in the circuit. These may be caused either by:
the clamp not making an adequate contact with the steel rim of the
the zinc tip being coated with cold, heavy or waxy oil, thus prevent-
ing zinc/water contact.
To operate the instrument:
lower the brass weight until it is well within the water layer, when
the needle should give full deflection and then withdraw slowly until
the oil/water interface is indicated by a return of the needle to zero;
place the brass weight into a sample can filled with clean water, and
lower the can and weight through the oily layer into the water layer.
When the meter needle deflects, lower the sample can away from the
brass weight and raise the weight slowly. The interface is indicated
when the needle returns to zero.
Another means of determining water levels in dirty ballast and slop tanks
is to use water-finding paste or ribbon. This is not recommended because
the paste colour and ribbon markings may be obscured or obliterated as
the tape is withdrawn through the oil.
(I) Tank Cleaning Chemicals
Chemical additives are available which will speed up the tank cleaning
process. They are useful in short haul voyages and in preparation for
repair ports. They are injected into the tank cleaning water, and are most
effective when used in water heated to about 65°C.
This temperature may have to be adjusted to take account of possible
damage to cargo tank coatings. It should be noted that the use of chemicals
is subject to internationally accepted safety recommendations applicable
to ships in which the tank atmosphere is uncontrolled during tank washings.
Oil residues accumulated in a slop tank from ballasting and tank cleaning
operations nearly always contain water-in-oil emulsions which are
extremely persistent. They can be broken by demulsifiers, a process
improved by the use of heat. The length of time heat is applied is more
important, however, than the temperature reached.
Responsibilities for complying with the procedures outlined in this
Guide rest with many people. By far the most important are the Charterer,
Owner, Master and Cargo Receiver.
The Charterer should include in the Charter Party a clause stating his
policy on oil pollution prevention, and requiring the tanker to operate in
accordance with the procedures set out in this Guide.
The Owner should:
(a) Co-operate fully with the Charterer in measures to avoid pollution.
(b) Provide detailed instructions for each of his vessels, based on the
principles and procedures set forth in this Guide, to satisfy variations
in construction and equipment.
(c) Leave Masters in no doubt that he supports these measures.
(d) Satisfy himself that the procedures set out in this Guide will be
correctly applied by means such as voyage reports.
(e) Arrange training programmes to instruct personnel in these pro-
The Master should ensure that his crew are fully proficient in carrying
out the procedures laid down in this Guide and that they apply them carefully
and conscientiously. He must notify the owner and seek his advice about cir-
cumstances which might impair his ability to comply with the procedures.
The Master should attach just as much importance to the correct keeping
of the Oil Record Book as of the ship's Official Log Book. Entries must be
accurate and should be made as soon as possible after any part of a Load on
Top operation has been completed.
The Master should regard the Oil Record Book as important evidence
in establishing whether or not he has contravened the regulations concerning
oil discharge, and should remember that the Book may be inspected by govern-
Entries may also be made in the Ship's Log Book to cover associated
entries in the Oil Record Book regarding time of commencement and com-
pletion of discharge, and any other pertinent details.
Cargo Receivers must be prepared to accept tank cleaning slops, or cargo
that has been commingled with retained oil residues. They must be prepared to
advise both shore and ship management of any requirements to segregate
retained slops from incoming cargo.
Water Effluent Cleanliness Requirements
The 60 Litres Per Mile Criterion
Any tanker sailing in normal weather conditions can keep within the
oil discharge rate limit of 60 litres of oil per mile of ship travel; oil discharged
at this rate quickly spreads to a thin film which disappears as detectable con-
tamination within two or three hours.
Contamination of the surface of the sea behind a discharging ship de-
pends initially on:
(a) the effluent discharge rate,
(b) the oil content of the effluent,
(c) the ship's speed.
The oil discharge rate in litres per mile is given by:
(ppm of oil in effluent) x (effluent discharge rate_in m 3 /hour)
(ship's speed in knots) X 1 ,000
Different combinations of effluent discharge rates, effluent oil contents and
speeds can clearly produce similar oil discharge rates. Consider, for example,
a large tanker travelling at 15 knots. Discharge of the clean part of the dirty
ballast water at, say, 8000 m a /hour and an oil content of 30 ppm gives an oil
discharge rate of 16 litres per mile. An identical discharge rate would be ob-
tained during the final slow discharge of the slop tank water as the oil/water
interface nears the pump suction, if the effluent discharge rate and oil content
were 80 m7hour and 3000 ppm respectively.
The 1/15,000 of the Total Cargo-Carrying Capacity
This requirement in association with the 60 litres/mile limit effectively
prohibits the discharge to the sea of bulk residues.
It must be appreciated that this limit of 1/15,000 includes the amount of
oil in the clean water discharged from the dirty ballast tanks and the oil content
of the water discharged from the slop tank. Due allowance must therefore be
made to avoid exceeding this total limiting amount.
The volume of oil in m 3 put to the sea in any effluent is given by:
(volume of effluent in m 3 ) x (oil content in ppm)
Load on Top Effluent Characteristics
The effluents from Load on Top operation have distinct characteristics :
(a) The discharge of the bulk of the settled dirty ballast. This effluent
is characterised by a high flow rate, large in quantity but of low oil
content. Oil content in good weather is typically around 30 ppm,
but higher oil content may be expected if there is substantial ship
(b) The discharge of the bulk of the settled water from a slop tank.
This is characterised by a moderate discharge rate, moderate quanti-
ties and moderate oil content. Oil content in good weather may be
typically around 150 ppm, but again higher oil content will occur
in heavy weather.
(c) The slow discharge of slop tank water as the oil/water interface nears
the tank suction. This effluent is characterised by very low discharge
rate, low quantity but probably of substantial oil content, par-
ticularly in the last stages just before visibly discernible oil dis-
colouration appears in the effluent. An average oil content of around
500 ppm is typical, rising sharply to double this or more as the
effluent becomes discoloured. Stopping further discharge at this
point will always leave a water bottom in the tank the depth of
which will vary with ship movement and the shape and structure
of the tank bottom.